16-04-2013, 03:06 PM
AUTOMATIC RECLOSING TRANSMISSION LINE APPLICATIONS AND CONSIDERATIONS
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INTRODUCTION
Various studies have shown that anywhere from 70%, to as high as 90%, of faults on most
overhead lines are transient [1, 2, 6]. A transient fault, such as an insulator flashover, is a
fault which is cleared by the immediate tripping of one or more circuit breakers to isolate
the fault, and which does not recur when the line is re-energized. Faults tend to be less
transient (near the 80% range) at lower, distribution voltages and more transient (near the
90% range) at higher, subtransmission and transmission voltages. [2]
Lightning is the most common cause of transient faults, partially resulting from insulator
flashover from the high transient voltages induced by the lightning. Other possible causes
are swinging wires and temporary contact with foreign objects. Thus, transient faults can
be cleared by momentarily de-energizing the line, in order to allow the fault to clear.
Autoreclosing can then restore service to the line. [6]
The remaining 10 - 30% of faults are semi-permanent or permanent in nature. A small
branch falling onto the line can cause a semi-permanent fault. In this case, however, an
immediate de-energizing of the line and subsequent autoreclosing does not clear the fault.
Instead, a coordinated time-delayed trip would allow the branch to be burned away without
damage to the system. Semi-permanent faults of this type are likely to be most prevalent in
highly wooded areas and can be substantially controlled by aggressive line clearance
programs.
Purpose
The purpose of this paper is to collect the various topics of protection that are associated
with reclosing and present them here for use in applying autoreclosing to transmission
circuits.
History
According to a report written by the IEEE PSRC in 1984 [1], automatic reclosing was first
applied in the early 1900s on radial feeders protected by instantaneous relays and fuses.
These schemes reclosed the circuit two or three times prior to lockout, with a 73% to 88%
success rate on the first reclose actions, and covered both radial and looped circuits,
predominantly at distribution voltages, but also including 154kV.
Jackson, et al [8], reported that high-speed reclosing (HSR) was first used by American
Electric Power System (then known as American Gas & Electric) in 1935 as a means to
defer construction of redundant transmission lines. System continuity was maintained on
these radial lines by rapidly reclosing a single line rather than providing a second, redundant
path for power to flow. Modern systems with single radial lines to transmit power from
one point to another are commonplace. It is more common to have a network with parallel
transmission lines. HSR is used more for maintaining system stability and synchronism
than for point-to-point continuity.
The development of high-speed breakers for transmission lines by the late 1930's led to
the application of high-speed reclosing (HSR) on these lines, resulting in improved system
stability. Probability studies of the insulator flashover were initiated to determine minimum
reclosing times that still permitted enough time for arc de-ionization. Early applications of
HSR on multi-terminal lines tripped all terminals and then reclosed the circuit breaker at
high-speed at one terminal. If this high-speed reclosure was successful, the remaining
terminals were reclosed with time delay to complete the through circuit. [1]
DEFINITIONS
Before discussing the issues involved in the application of autoreclosing schemes, it is
useful to define some of the terms in common usage. The majority of these definitions are
taken from reference [3], IEEE Standard Definitions for Power Switchgear, IEEE Std.
C37.100-1992.
Several of the terms defined below are illustrated in Figure 1, which shows the sequence of
events in a typical autoreclosing operation, where the circuit breaker makes one attempt at
reclosure after tripping to clear a fault. Two conditions are shown: a successful reclosure in
the event of the fault is transient, and an unsuccessful reclosure followed by lockout of the
circuit breaker if the fault is permanent. [2]
Application of Autoreclosing on Transmission Systems
A primary concern in the application of autoreclosing, especially on EHV-rated lines and
higher, is the maintenance of system stability and synchronism. This is normally done
through the application of high-speed tripping and autoreclosing. The problems involved
with maintaining stability on these lines when autoreclosing during a fault on the line depend
on the characteristics of the system - whether it is loosely connected, for example,
with two power systems connected by a single tie line, or, conversely, highly interconnected,
in which case maintaining synchronism during autoreclosing is much easier.
The intent of autoreclosing on transmission and subtransmission systems, other than the
maintenance of stability, is to return the system to its normal configuration with minimum
outage of the line with the least expenditure of manpower. System restoration becomes
increasingly important when applied to lines that interconnect systems and are critical for
reliable power exchange between the systems. Individual utility policy and system requirements
dictate the complexity and variety of automatic reclosing schemes in service today.
System Stability and Synchronism
Any unbalance between generation and its load initiates a transient that causes the rotors
of the synchronous machine to swing because net accelerating (or decelerating) torques
are exerted on the rotors. If these torques are large enough to cause some of the rotors to
swing far enough, one or more machines may slip a pole and synchronism is lost. In
order to ensure stability, a new state of equilibrium must be reached before this can happen.
Loss of stability can be caused by a severe generation unbalance (e.g., excess generation
due to loss of load). Figure 3 shows how the rotor angle of the machines will increase. If
the angle differences between the machines do not change significantly, synchronism will
be maintained and the machines will eventually settle to a new angle (a). If the machines
are separated by large angles, they will continue to drift apart and the system will become
unstable (b).